CONTINENTAL DYNAMICS - ROCKY MOUNTAIN PROJECT (CD-ROM) - AN INTERPRETATION OF THE HIGH - VELOCITY LOWER CRUSTAL LAYER ALONG THE CD-ROM '99 SEISMIC REFRACTION/WIDE-ANGLE REFLECTION PROFILE

The Continental Dynamics - Rocky Mountains Project (CD-ROM '99) is a collaborative interdisciplinary study involving ~14 American universities and the University of Karlsruhe, Germany and focuses on Precambrian features and their effects on Phanerozoic deformation. One of the major field efforts in the CD-ROM project took place during August, 1999. The University of Texas at El Paso and the University of Karlsruhe, with the assistance of several other institutions, collected data along a ~950 km long seismic refraction/wide-angle reflection profile from about Fort Sumner, New Mexico to the Gas Hills, Wyoming. Station spacing was nominally 800 m using ~600 instruments during two deployments. Eleven shots were fired ranging in size from ~900 to 4500 kg and were nominally spaced at ~100 km intervals along the profile. The profile crossed such geologic features as the Jemez lineament, the Colorado mineral belt, and the Cheyenne belt (a prominent Proterozoic suture).

Velocity modeling indicates crustal thicknesses ranging from ~45 to 50 km in New Mexico and Colorado. In northern Colorado the crust begins to thin from ~50 and reaches ~40 km in Wyoming, near the Cheyenne belt. A high-velocity lower crustal layer of about ~10 km thickness is evident in the Southern Rocky Mountains - Great Plains (SRM-GP) portion of the model. The velocity of this layer ranges from 7.2 to 7.4 km/s, a value that is consistent with a composition of mafic garnet granulite. One interpretation of this high-velocity lower crustal layer is that it originally formed during assembly of the Proterozoic terranes. Magmatic underplating at 1.4 Ga may have increased the thickness of this layer beneath the SRM-GP. We are currently evaluating whether subsequent Phanerozoic events have inflated, attenuated, or completely removed this layer in the Rocky Mountains.